The parameterized chemistry and transport model (PCTM) is one of several numerical tools which the Atmospheric Chemistry and Dynamics Branch uses to study the evolution of trace compounds in the Earth's atmosphere. Please browse through these pages to become familiar with the PCTM and the scope of issues to which it is applied.
Each step of the integration is accomplished with two processes. Advection is done first and produces an "intermediate" consituent configuration, x*. In prediction form, Equation 1 is written in terms of x* as
Second, the parameterization of the photochemistry is applied to x* by
The coefficients P and L are specified in a variety of ways. The simplest and most computationally efficient method involves reading tables of P and L where each is a function of, say, latitude, pressure, and time. For surface source gases, the surface exchange in kg m-2 s-1 is converted to ground-level production rates based on the volume of each respective surface layer grid box. For many short-lived tropospheric gases, the loss frequencies are determined by multiplying published reaction rates by the local number density of the particular reactant.
The subgrid-scale transport due to convection process and turbulence in the planetary boundary layer (PBL) is included in the PCTM. The parameterizations of these subgrid-scale processes are done in the GCMs. The output of the cloud mass flux and turbulence diffusion coefficients from the data assimilation models or GCMs are used by the CTM to compute the subgrid-scale transport of the tracers.
The PCTM is run in the so-called offline mode with the dynamics supplied by archived meteorological datasets. The winds, temperatures, surface pressures, and cloud mass flux and detrainment are most often provided by the Goddard Earth Observing System (GEOS) Data Assimilation System (DAS). This reflects our emphasis on model validation through the use of observed constituent mixing ratios.
Recently, the model has been improved in the transport as well as in the subgrid parameterization in order to enhance the accuracy and to ensure mass conservation (see New Development).